The present invention relates to force applicator devices including indicators for indicating the force applied. The invention is particularly useful with respect to surgical tool spreaders for applying an extension force to a part of the human body and for providing an indication of the force applied. The invention is therefore described below with respect to this application, but it will be appreciated that the invention, or various features thereof, could advantageously be used in other applications as well.
Many surgical procedures and other medical treatments, especially in orthopedics, involve the use of force. For instance, scoliosis, which is a lateral curvature of the spine, is corrected through external bracing (Milwaukee brace) or surgically (Harrington rod procedure). In the first one, a longitudinal force is applied between the head and the pelvis together with lateral pressure. In the latter, used for more severe cases, extension forces are exerted directly on the spine. In both cases, the magnitude of the applied force is of vital importance to the success of the treatment. Insufficient force will not produce the desired effect, while excessive force may cause accidents such as fracture of the posterior elements of the vertebrae or paraplegia, which appears to be a consequence of a vascular lesion.
The Harrington rod procedure, today the more common of the two above surgical techniques, uses two hooks and a rod with a series of circumferential grooves. One hook rests against a shoulder at one end of the rod, while the other may slide along the rod. The two hooks are inserted into the posterior elements of the spine at the two ends of the curve. A spreader is used to move one hook along the rod and distract the spine.
During this operation, a considerable force may be applied to the spine. In order to avoid any damage, it has been recommended to have a safety limit of 30 to 40 Kg axial force. Two devices have previously been proposed for indicating the force. One includes a mechanical indicator provided by cutting one handle of the instrument and connecting it back through a leaf spring, using the deflection of the spring as a measure of the force; however, the flexibility of the spring increases the flexibility of the handle, which is not always desired by the surgeon. The other device includes resistance strain gages attached to the spreader as an electrical force transducer; this method, however, involves an expensive measuring system and inconvenient wiring, and moreover, makes direct reading of the force by the surgeon very difficult because the force indication is at some distance from the operation site.